[LU-13412] fix read if readahead window smaller than rpc size

• Type: Improvement
• Resolution: Fixed
• Priority: Minor
• Fix Version/s: Lustre 2.14.0
• Assignee: Wang Shilong (Inactive)
• Reporter: Wang Shilong (Inactive)

Description

Readahead always try to align readahead with RPC size, but this
could introduce a problem if readahead window is smaller than RPC size.

With current codes, it will fallback a lot of 4k read because most of readahead stop, fix this to align with readahead window rather than RPC size in this case.